Elastin is an amorphous protein present in the elastic fibers of tissues such as arteries, blood vessels, skin, tendons and elastic ligaments, the abdominal wall, and lungs. Unlike other fibrous tissues like collagen, elastin is unique in that it may be stretched to over 150 percent of its original length but it can rapidly return to its original size and shape. This property of elastin provides tissues that incorporate it, the required ability to resume their original form after stretching due to blood flow, breathing, or bending. Like collagen protein, elastin contains about 30% glycine amino acid residues and is rich in proline. Elastin differs from collagen in that it contains very little hydroxyproline and no hydroxylysine. Elastin has a very high content of alanine and also contains two unique amino acids isodesmosine and desmosine. These amino acids are believed to be responsible for elastin's ability to return to its original shape after stretching.
Tropoelastin is a soluble precursor of elastin; it is a peptide with a molecular weight in the range of 70-75 kDa, it is synthesized by dermal fibroblasts and secreted in association with the 67 kDa elastin binding protein (EBP). EBP acts as a molecular chaperone protecting the highly hydrophobic tropoelastin molecules from intracellular self-aggregation and premature degradation and facilitating their proper assembly on the microfibrillar scaffold in the extracellular space. In the arterial tissues tropoelastin is produced and secreted into the extacellular space by smooth muscle cells; in other tissues it is produced in cells, like fibroblast cells, and is also secreted into the extracellular space. In these cells tropoelastin is synthesized by ribosomes in the rough endoplasmatic reticulum and processed by the Golgi apparatus. The soluble tropoelastin molecules secreted (often referred to a proelastin before secretion) into the extracellular space synthesize to form Elastin filaments and sheets via cross linking of the tropoelastin molecules primarily by crosslinking of lysine amino acid residues to form desmosine and isodesmosine. Mature elastin is amorphous and contains many cross links which makes it nearly impossible to solublize.
The resiliency of skin is maintained by elastic fibers in the extracellular matrix (ECM). These ECM components are organized into a networks of rope-like structures and composed of two major components: an amorphous core, consisting of extensively crosslinked elastin which makes up the bulk (>90%) of the fiber; and the 10-12-nm microfibrils made up of several distinct glycoproteins.
In various tissue or biological functions, inelastic collagen fibers may be interwoven with the elastin to limit stretching of the elastin and prevent tearing of elastin comprising tissue. Elastic fibers may also contain glycoproteins as microfibrils, which may serve to organize tropoelastin molecules secreted into the extracellular space for later crosslinking. Examples of such glycoproteins include laminin, which is a large glycoprotein and a major component of basement membranes and is made by all epithelial cells, and fibronectin which is a cell-surface and blood glycoprotein involved in a variety of cell surface phenomena.
Combinations of components of the extracellular matrix have been incorporated into cosmetic compositions. Elastin is insoluble due to its high degree of cross linking at its lysine residues and also because of its high content (about 75%) of hydrophobic amino acids (Gly, Val, Ala, Pro). In some instances, normally cross-linked insoluble elastin (i.e., insoluble in water, organic solvents, and physiological fluids such as saline and blood) is rendered soluble using a variety of chemical and enzymatic methods to cleave insoluble elastin protein and form smaller peptide fragments.
The human skin consists of two layers; a superficial layer called the epidermis which is epithelial tissue and a deeper layer called the dermis that is primarily connective tissue. These two layers are bound together to form skin which varies in thickness from less than about 0.5 mm, to 3 or even 4 millimeters. The main types of proteins that make up the matrix include collagens, Elastin, fibronectin and laminin. Normal elastic fiber assembly is visualized as a spider web spanning the dermis. Exposure of the skin to ultraviolet and visible light from the sun, wind, and chemicals leads to loss of moisture in the epidermal layers and degradation of the elastin present in the skin. Loss of elasticity in skin primarily occurs because of an over-production of poorly assembled elastic fibers induced by exposure to sunlight. These poorly assembled elastic fibers can be visualized as “clumps” in the dermoepidermal junction and papillary dermis and is commonly referred to as solar elastosis. These effects, result in loss of skin elasticity, tone and texture, are collectively referred to as aging of the skin. Loss of elasticity in elastic tissues such as arteries is mainly due to calcification and glycation of elastic fibers.
Stretch marks, also known as striae gravidarum, or ‘striae’, are the lines that appear on the skin which are caused by a breaking of the elastic layer of the epidermis. Skin is composed of two layers, as described above. Stretch marks occur in the dermis, the resilient layer that helps skin retain its shape. As a result of this layer being constantly stretched over time it breaks down and becomes less and less elastic and the small connective fibers within it break. Striae may take many forms and colors, from almost invisible tiny lines to deep red lines. Both men and women suffer from this imperfection, although the majority of those who suffer from this skin condition are pregnant women. Stretch marks may occur on the breasts, the upper arms, the buttocks, thighs and across the entire abdomen. Stretch marks may appear in patients after a breast augmentation or other cosmetic surgery procedures.
Stretch marks are wide purplish red lines on the skin which appear on different areas of the body as the pregnancy develops with ever increasing stretching of the skin. Stretch marks are most noticeable in the beginning when they are raised, pink, reddish brown or dark brown lines that later turn to a brighter purplish or a brisk red. In the pregnancy context, hormonal changes help the skin and ligaments to relax and stretch. During the postpartum period, these red lines can turn to silver. They then gradually flatten and fade out to a less noticeable silvery color. Eventually they will become a few shades lighter than the natural skin tone.
One explanation for the formation of stretch marks is best explained through pregnancy related stretch marks. As the uterus becomes larger and the breasts develop, the skin in certain women is not able to keep up with this rapid growth. The fibrous and elastic tissue in the skin is damaged and stretch marks form. As the stretching process increases, more scar tissue is accumulated and the stretch marks increase in width.
Stretch marks are not a very well understood skin condition. There are some known factors that determine if one is susceptible to the condition of stretch marks. Stretch marks have been linked to genetics and some estimate that about 15 to 20 percent of the population is genetically “stretch mark free.” Ethnicity also factors into whether one will develop stretch marks. Darker skinned people tend to get less stretch marks than fair-skinned people.
Some dermatologists have suggested that stretch marks are often mistakenly blamed on the rapid stretching of the skin associated with such life events as pregnancy and growth spurts. These dermatologists have stated that stretch marks are the result of an increased level of circulating glucocorticoids throughout the bloodstream. This hormone, secreted by the adrenal glands, becomes elevated during pregnancy, adolescence, with obesity, weight lifting and Cushing's disease.
These dermatologists speculate that the glucocorticoids responsible for the development of a stretch marks affect the dermis by preventing the fibroblasts from forming collagen and elastin fibers, necessary to keep rapidly growing skin taut. This creates a lack of supportive material, as the skin is stretched and leads to dermal tearing. The epidermal cells are also affected, so the epidermis becomes thin and flattened, allowing for more visibility of the defects below.
Striae are a disfiguring skin condition normally associated with obesity, pregnancy and adolescent growth and are a pathological symptom of Cushing's, Marfan's and Ehlers-Danlos syndrome syndromes, diabetes mellitus6 and long term use of topical and systemic steroids. The pathogenesis of stretch marks is as yet unknown but is thought to be a response to minimal or excessive stretching of skin1. Previous histopathological analysis of stretch marks demonstrated loss of dermal papillae and epidermal changes including atrophy, loss of rete ridges and conflicting observations regarding collagen and elastic fibers. More recent studies indicate that steady-state mRNA levels of collagens, elastin and fibronectin are decreased. Studies aimed at quantifying protein levels of elastin and fibrillin in stretch marks further confirmed decreased levels of both.
In 1982 Pieraggi and co-workers proposed that stretch marks are perhaps a consequence of fibroblast dysfunction, based upon their histological observations that fibroblasts were globular, quiescent, and appeared to lose all signs of fibrillar secretion in stretch marked skin. This implies that perhaps dermal fibroblasts in stretch marks have, temporarily or permanently, impaired proliferative and synthetic capabilities with regard to extracellular matrix. Elastic fibers are perhaps the most under-studied extracellular matrix components in skin and potentially a key player in stretch mark formation as there is no strong evidence that collagen levels are affected.
Stretch marks may be induced by excessive mechanical stretching of skin to the point of rupturing dermal elastic fibers and that local fibroblasts are unable to adequately repair or replace these ECM components that are solely responsible for the resilience of skin. Since many patients with stretch marks do not display any obvious signs of known genetic diseases permanently affecting connective tissue, these lesions may develop as a consequence of acquired metabolic disturbances that significantly diminish the reparatory abilities of dermal fibroblasts.
A better understanding of the development of stretch marks is necessary. It is generally understood that stretch marked skin contains lower collagen and elastin content than normal/healthy skin, but a biological analysis of stretch marked skin could lead to the development of enhanced therapeutic treatments.
Not all women will develop stretch marks associated with pregnancy. There are some factors that women may control to reduce the chance of stretch marks. Dry skin tends to be less elastic than well nourished or oily skin. Thus, dryer skin tends to be more susceptible to stretch marks. Part of keeping skin healthy and well-hydrated is dietary. Healthy skin will stretch better and will also repair itself quickly with little damage. Rapid or excessive weight gain will only compound the problem of stretch marks and likely cause more.
Some commonly known preventative measures to reduce the appearance of stretch marks include the follow therapies: massaging skin with a glove or a massage brush to increase circulation; and eating foods that contribute to the overall health of your skin, such as those high in vitamins C and E, zinc and silica. Zinc is especially important because it is linked to cellular growth. Exercising regularly and slow and steady weight gain during pregnancy also may prevent the formation of stretch marks. Several active ingredients have been identified as beneficial to the prevention of stretch marks including emu oil and vitamin E oil. Vitamin A is also a good overall moisturizer, but not as effective as tretinoin, or Retin-A, which helps exfoliate the skin and form healthy new cells.
Several treatments are also known to reduce the appearance of stretch marks. Stretch marks may never entirely be diminished. The sooner one begins treating stretch marks, while they are still reddish or purple, the better likelihood of diminishing their appearance. Once they flatten down and become more silvery they are more difficult to treat. There are several treatments on today's market that range from all-natural creams to invasive surgical measures.
Natural creams and oils include for example, Vitamins A and E and emu oil, are all natural, non-invasive treatments. Retin acid cream or glycolic acid are also known, which act to slough off the top skin layers and stimulate the skin. Another promising new treatment that can help the reduction and prevention of stretch marks include microdermabrasion. In this procedure, a dermatologist administers a stream of fine, chemically inert crystals onto the skin to exfoliate the outer most layers of the epidermis. Laser therapies have also been used to eliminate the appearance of stretch marks. A tiny pulse is emitted from the laser and is absorbed by the blood vessels in the affected area below the skin. Blood vessels rupture, bruise and recover in an accelerated healing process. Reducing contact with the sun and keeping the skin soft and moist have been general suggestions for patients with stretch marked skin.
Finally, cosmetic surgery, which is usually used as a last resort, may be used for the most severe scarring from stretch marks. A tiny incision is made along the length of the stretch mark and the affected area is removed and then stitched together.
There remains a need to identify patients who have a high incidence of stretch marks. There remains a need to identify the causes of stretch marks and how the skin's biological content changes from normal to stretch marked skin. There remains a need to objectively identify the likelihood of the development of stretch marks. There also remains a need to develop treatments and protocols, which may stimulate elastogenesis and the production of the extracellular matrix components in damaged or stretch marked skin. Additionally, stretch marked skin may provide a basis for therapies relating to the stimulation of cellular proliferation and elastogenesis. The use of stretch marked skin which contains a loss of elastin fibers may be used to determine the effectiveness of several potential therapies which may generate or stimulate cellular function in the skin.
Aspects of the present invention delineate possible functional differences between dermal fibroblasts derived from biopsies of unaffected skin regions of patients with stretch marks and fibroblasts derived from skin of normal age-matched individuals. Aspects further provide methods for determining functional differences between fibroblasts derived from these two groups, thereby allowing for the prediction of individuals that may be predisposed for development of striae distensae in skin, challenged by otherwise physiological stretch during the pregnancy. Aspects of the invention provide preventive treatment(s) for individuals diagnosed with predisposition to stretch mark development and eventual treatment of fully developed lesions.